The present invention relates to a particularly environment-friendly and economic process for preparing transparent N,N'-dimethylperylene-3,4,9,10-tetracarboxylic diimide pigments for pigmenting materials of high molecular mass.
The pigmentation of automotive finishes, especially the metallic finishes, requires pigments which, with high transparency and a pure hue, produce strongly colored and highly lustrous coatings. The paint concentrates (millbases) and the paints themselves must be of low viscosity and should not exhibit any pseudoplasticity. A further requirement is for very good fastness properties, especially weather fastness.
N,N'-Dimethylperylene-3,4,9,10-tetracarboxylic diimide (dimethyl-perylimide), as an organic red or chestnut pigment (C.I. Pigment Red 179, C.I.No. 71130), has been in use for a long time, and both transparent and hiding forms of the pigment are known. Depending on the production conditions, red or chestnut-colored pigments are obtained. The following processes are described for its production:
DE-B-21 53 087 (GB-A-1 370 433) describes the reaction of perylene-3,4,9,10-tetracarboxylic dianhydride with monomethylamine at from 120 to 150.degree. C. using high concentrations of the dianhydride. The crude dimethylperylimide pigments obtained in the condensation are isolated, dried and then converted to the final pigment form by an intensive grinding and kneading operation with salt followed by renewed drying. Following each stage of the process the pigment is isolated, so that there are large amounts of wastewater produced. However, the process does not yield a transparent dimethylperylimide pigment suitable for use in metallic finishes. DE-B-2 504 481 describes a process for producing transparent dimethylperylimide pigments by condensing perylene-3,4,9,10-tetracarboxylic dianhydride with at least 4 times the molar amount of monomethylamine. The excess monomethylamine is separated from the pigment suspension by filtration and can be distilled off from the mother liquor using steam. The filtration of the methylamine-containing suspension causes considerable technical problems, since because of its severe odor monomethylamine must not be allowed to pass into the environment. An additional distillation unit is required for working up the methylamine-containing mother liquor. Transparent pigments are obtained only in high dilution.
EP-A-0208 266 discloses the production of a hiding dimethylperylimide pigment. In this process, dry, finely divided, crude dimethylperylimide pigment with a particle size of less than 0.04 .mu.m are subjected to a recrystallizing grinding process in a mill or disperser in the presence of an organic solvent, and the dimethylperylimide pigment is subsequently isolated.
EP-A-0 088 392 describes a process for producing transparent dimethylperylimide pigments by reacting peracid with at least 4 times the molar amount of monomethylamine. The excess monomethylamine has to be liberated from the mother liquor using sodium hydroxide solution and separated off by distillation. This produces considerable amounts of salts which load the wastewater.
EP-A-0 318 022 describes a process for preparing hiding dimethylperylimide pigments by reacting peracid with at least 2 times the molar amount of monomethylamine. The crude pigment obtained after the condensation is ground on a stirred ballmill and then finished with solvents. It is not possible to obtain transparent pigments by the condensation, beadmilling and finishing conditions described therein.